However, that does not mean that we are immune. Here at the clinic we have seen a rash of flea cases in the past few weeks. Why? Probably a combination of sheer (bad) luck and the cool wet weather. Normally we see the highest caseload of flea infestation during early spring. The early spike tapers off, and we don't see much through the summer and fall.
Dogs and cats acquire fleas from the environment. Fleas can be found anywhere animals congregate. This can be the dog park, public park, your back yard, or even your house. So even if your dog only goes outside to go to the bathroom, your dog can acquire fleas brought there by some other animal (think feral cat or neighbor's dog). Once on your pet, the fleas begin setting up in your house. If you see a flea on your pet, you can assume there are hundreds or even thousands of flea eggs and larva in your carpets and pet bedding.
If your pet (and your house) already has an infestation, it has to be treated aggressively. Half measures will lead to a continuing infestation and greater long term expense. The most important aspect of treating an established infestation is; You must treat all animals in the household at the same time. If you dog-sit your cousin's dog once a month, he must also be treated. Even though you see fleas on your dog but never your cat, you must treat your cat. An untreated animal will act as a reservoir that will continue the infestation.
My dogs came down with a case of fleas a couple years ago. I suspect they picked them up at the public park across the street. As such, they are on flea prevention for the rest of their lives. It's just part of the cost of having a healthy pet.
Extra Credit: Fleas are an interesting parasite that have had a profound impact on human history. There are many different species of fleas. Each one is relatively host-specific but most are also opportunistic. Fleas that commonly infest dogs and cats are Ctenocephalides canis and Ctenocephalides felis. These fleas will prefer to bite your pet, but will occasionally bite you if the opportunity presents itself. Humans have their own flea (Pulex irritans) which has a fairly wide host range.